Hi, I have an ASUS RT-AC 1200 RU router, can I install firmware from ASUS RT-AC1200 V2/RT-AC 1200 on it?
It's hard to say, if the same SoC is used (MT7628), you might be in luck, but flash layout might differ. v1 and v2 seem pretty similar, at least. I would first make sure flash layout is identical, so you aren't overwriting any crucial data (MAC addresses, bootloader configuration, radio calibration data etc). There is no documented way to run OpenWrt from RAM for this device (which allows you to test without touching the flash).
Ok, I also looked at the fact that the fabric.bin file is available for V2, this may mean that if you can also download the firmware in the web interface of my router, then will it work?
There's no guarantee.